1-1 An Overview of Pentacene-based Thin Film Transistors
Recently, organic thin-film transistors (OTFTs) have received great attention due to their low-cost and large-area array application. In numerous organic materials, pentacene is promising candidate due to its high hole mobility. Pentacene is made up of five benzene rings as shown in Fig. 1-1.
In previous studies, there are many superior groups to promote the electrical characteristic of pentacene-based thin-film transistors such as field-effect mobility, subthreshold slope, Ion/Ioff , and low operation voltage.
OTFT arrays to drive liquid crystal (LC) [1] [2] or organic light emitting diode (OLED) [3] which showed full-color moving pictures had been demonstrated. These fabrication processes and the handlings of the backplanes should be carried out in ambient air to enable simple inexpensive production. In these reports, OTFTs were encapsulated by passivation layer to avoid exposing to oxygen or moisture in air, and to avoid damage from the subsequent LC or OLED process. However, even when devices are encapsulated or operated in an inert environment, OTFTs are known to suffer from bias stress effect (BSE) that causes significant threshold voltage shift.
1-2 Operation of OTFTs
A thin film transistor is composed of three basic elements: (i) a thin semiconductor film; (ii) an insulating layer; and (iii) three electrodes (source, drain and gate). Fig. 1-2 show two kinds of standard OTFT device structure. Fig. 1-2 (a) is the top-contact device and Fig. 1-2 (b) is the bottom-contact device, respectively. The general
traditional MOSFET are usually operated in inversion mode while the OTFTs are generally operating in accumulation mode.
Since the pentacene is a p-type semiconductor, negative bias is applied on the gate to turn on our OTFTs. The voltage-drop across dielectric causes the energy band bending in the organic semiconductor and additional positive charge carriers will accumulate at the interfaces. The dielectric serves as a capacitance and can store charges. Then we apply a drain bias to drive the accumulated charges from source to drain and from the drain current. The conduction is determined by the field effect mobility (μFE) which represents charges’ driving ability by the electrical field.
In general, we can divide the operation of OTFTs into two regions: linear region and saturation region. If we add gate bias at turn on state, beginning with small drain voltage, OTFTs are operated in linear region, as given drain become larger the drain current will gradually saturate and into saturation region.
Understand how OTFTs normally operate, parameters such as the threshold voltage, field effect mobility can be extracted according to the measured electric characteristic. In addition, how environment effect devices can also be told by analyzing abnormal changing of these parameters.
1-3 Instability of OTFTs Operation
The bias-stress effect in OTFTs had been studied by using different organic active materials or different gate insulators on different device structures [4]. It was found that, for p-type OTFTs under DC stress, positive gate bias stress caused a positively-shifted VT and negative gate bias stress caused a negatively-shifted VT. The BSE was reversible by removing gate bias or by applying opposite polarity gate bias. Light irradiation also enhanced the reversal process.
Charge trapping, ion migration, charged-state creation and the formation of bound hole pairs (bipolaron) are several proposed mechanisms to explain the BSE [5]. Charge trapping and ion migration were found to be dominant mechanisms in OTFTs with an organic dielectric [6]. When using thermally-grown SiO2 as the gate dielectric to study OTFTs reliability, charged-state creation is usually believed to be responsible for ΔVT. John E. Northrup and Michael L. Chabinyc used density functional calculation to simulate defect states generation in pentacene film and found that it was due to the formation of oxygen- and hydrogen-related defects such as C-H2, OH, and C-HOH in organic semiconductors [7]. Gu et al. also studied the response time of the defect states in pentacene. Long-lifetime deep electron traps were proposed to explain the hysteresis effect in pentacene-based OTFTs [8].
1-4 Defect Generation Mechanism
Until now, device reliability issue has been a greatest barrier to realize the organic electronic application. Even when devices are encapsulated, the threshold voltage (VT) tends to shift under continuous bias and the field-effect mobility degrades after prolonged storage in normal environment [6]. The device threshold voltage shift (ΔVT) is generally attributed to hole/electron trapping in the interface between pentacene and dielectric. Although the field-effect mobility degradation mechanism is not clearly understood, the permeation of H2O and O2 in pentacene film is the usually proposed mechanism. These two phenomena seriously strict the organic TFTs application ranges.
1-5 Threshold Voltage Shift Mechanism
The ΔVT of OTFTs is believed due to the carrier trapping by the defect states.
However, there are only a few explanations on the micro process of the defect creation,
into two kinds: negative bias stress and positive bias stress.
First, micro process of defect creation under negative bias stress is introduced. The formation of bipolaron proposed by R. A. Street et al. (Phys. Rev. B, vol.68, 085316, 2003) is one of the plausible mechanisms. The deep states slowly trap holes to form bipolarons. The formation of bipolarons would cause the ΔVth due to the reduction of mobile holes. The reaction can be expressed as:
h h+ →( hh )BP
The other possible mechanism was proposed by John John E. Northrup et al. (Phy. Rev.
B, vol.68, 041202, 2003) They studied the formation of hydrogen- and oxygen-related defects (C-H2, OH, and C-HOH) in pentacene film based on the density functional calculation. The defect creation reactions were given as follows:
and
22 16 22 14 22 15 22 15
2h++C H O C H+ →(C H O)+ +(C H )+
When the pentacene film is in a hole-rich environment, both these two reactions tend react to the right-hand side and produce positive-charged states that cause the ΔVTH. Either bipolaron formation or hydrogen-, oxygen-related defect creation, these studies need more experimental results to support their theories. Both mechanisms assume that the reaction rate is proportional to the carrier concentration.
However, compare with negative bias stress effect, there are fewer studies focused on positive bias stress effect. Applying a prolonged positive bias to the device usually causes electrons trapping in the channel and a threshold voltage shift forward positive bias. After removing the positive bias, the recovery of trapped electrons can be observed and the device threshold voltage comes back to the original value. Until now, the micro process of electron trap generation under the positive bias stress is not
+
+ + + →2( )
2h C22H16 C22H14 C22H15
discussed in detail.
1-6 Hysteresis Mechanism
The hysteresis effect is often found in OTFTs, such as polymer gate dielectrics with hydrophilic and polar groups. The large hysteresis prohibits the OTFTs from applying to a circuit and a switch device for active matrix displays. The threshold voltage in the semiconductor accumulation mode is given as [9]:
V qn d
C V
, by this equation we could get a change in VT between forward and backward scan then cause hysteresis effect we observe, there are three factor could affect the VT change, 1. no changes (due to charge trapping).
2. Ci changes (charge injection form gate into dielectric or slow polarization).
3. Vfb (structural changes in the semiconductor).
Except the reason about above mathematical equations, the hysteresis effect formation can be classified to three general mechanisms [10]:
1. Channel/dielectric interface induced effect
2. Residual dipole-induced effect caused by slow polarization in the bulk organic dielectric
3. The effects of charges injected from gate electrode.
The OTFTs with polymer gate dielectrics might have anyone mechanisms to affect operation stability. Until now, it’s not clearly for saying hysteresis phenomenon for polymer dielectrics how to form and how to influence the device operation such as memory effect and bias stress effect.
1-7 Polymer Gate Dielectrics
Polymer gate dielectrics can offer application such as large area, simple process, and flexible displays. The interface between organic semiconductor and insulator has strongly relation to electric properties like carrier mobility, threshold voltage, and charge trapping.
PVP (Poly (4-vinyl phenol)) has been reported to be the best polymer gate dielectric. Because it offers high mobility, low leakage, cross-link ability [11], and show the potential for photo-pattern possibility [12]. But its stability such as hysteresis and bias stress effect are need to overcome for application of display circuit. The hysteresis is associated with dipole polarization in the dielectric bulk related to gate voltage, charge trapping in the dielectric due to gate inject charge [13]. Recently, several groups investigated hysteresis and bias stress effect problem of pentacene-based TFTs with bilayer gate dielectrics like PVP and SiO2 [13-16]. The thickness of PVP represents the amount of residual OH group could cause slow polarization induced hysteresis [13]. There have been many attempts to reduce the concentration of hydroxyl groups in the polymer gate dielectrics by cross-linking them with curing agents, both thermally and photochemically [13-17]. The cross-linking agent ration to PVP is also discussed, increase PMF amount would decrease hole mobility and reduce hysteresis effect [17]. By deep curing PVP or in a vacuum chamber could found polarization effect decrease.
PVP-co-PMMA (poly(4-vinyl phenol)-co-(methyl methacrylate)) has been reported to fabricate hysteresis-free pentacene TFT [19]. The polymer dielectric looks like better than PVP, due to its low leakage current, hysteresis-free, high on/off ration, and photo-crosslink ability. But its bias stress effect is not discussed and hydroxyl groups in the polymer how to influence less to the OTFTs.
1-8 OTFTs for Gas-sensing
For the next generation of sensor applications of medical diagnostics, food monitoring, and chemical or biological warfare are desirable. Portable, low cost, and low power-consumption will be newly demands. The OTFTs should be an adequate candidate due to its solubility and simple fabrication process. In recent, OTFTs are proposed to do as gas sensors. When OTFTs exposed to gaseous species, five parameters: turn on current (Ion), turn off current (Ioff), threshold voltage (VT), field-effect mobility (μFE) and the subthreshold-swing (S.S), are used to estimate the gas interaction with OTFTs [20].
Analyzing the chemical composition of human breath helps people to examine their health conditions. A breath-testing for alcohol has been used to detect ethanol concentrations in the ppm range since 1960s and it has been applied extensively in alcohol test of car drivers. Except the main components such as nitrogen, oxygen, water vapor and carbon dioxide, more than 200 gaseous molecules (organic or inorganic) (Conkle, J. P. et al, 1975) exist in human breath. These gaseous species produced in human body reflect normal physiological biochemical processes or pathological conditions such as gastric ulcer, liver disease, cancer, or renal failure. Increased concentrations of some species (target analytes) have been found to correlate with certain diseases as listed in Table 1(a). Sensing the unusual concentration of these target analytes in human breath can be a helpful reference to detect human body conditions. In other words, if sensors can be developed to detect and trace these target analytes in patients’ breath, non-invasive diagnostic of above diseases can be realized.
1-9 Gas Sensing Mechanisms for OTFTs
Morphology effect is a critical sensing mechanism, it could decide gaseous diffuse into channel quickly or slowly. Grain boundaries play an important role in OTFT sensing. The grain size was varied by changing the substrate temperature during deposition, deposition rate surface roughness, and surface energy of dielectrics.
In the organic semiconductor, both grains and grain boundaries could be affected by the analyte. Due to their dipole, the analyte molecules bound on grain boundaries will trap the mobile charge carriers from channels.[21-22] Meanwhile the analyte interacting with the semiconductor grains will result in excess holes through chemical processes that are not completely understood.
1-10 OTFTs for Ammonia Gas Sensing
According to medical reports (C. Shimamoto et al, 2000), breath ammonia levels are significantly higher in cirrhotic patients (0.745 ppm) than in controls (0.278 ppm).
Ammonia is an important index for uremia and chronic liver disease as listed in Table 1(b).Patients who have renal failure even exhale 4.8 ppm ammonia in their breath.
Current ammonia sensors such as polyaniline sensors, metal oxide sensors, catalytic sensors, and optical analyzers have disadvantages such as high operation temperature, low sensitivity or high cost (G.K. Prasad et al., 2005; Björn Timmer et al., 2005). OTFT is promising to be a non-invasive, inexpensive, portable and disposable diagnostic device because of its low cost fabrication process and high sensitivity to gas molecules.
1-11 Motivation
Organic thin-film transistors (OTFTs) have been extensively explored due to their potentials for low-end electronic applications: drivers for flexible displays, complementary circuits, and radio-frequency identificationtags (RFID). PVP is a well-known polymer gate dielectric due to low leakage and high carrier mobility, but owing to hydroxyl group results in worse bias stress effect and hysteresis show. Make it’s hard to apply for flexible display driving, etc. On the contrary, there is less gate bias stress effect and hysteresis-free for OTFT with PVP-PMMA dielectric which also has hydroxyl group. In previous work, the cross-linking agent ration to PVP is also discussed, increase PMF amount would decrease hole mobility and reduce hysteresis effect [17]. There is high hole mobility for OTFTs with PVP dielectric might due to hydroxyl group dominating. But hydroxyl groups influence on PVP and PVP-PMMA devices differently in ambient air. Above all, it’s necessary to realize polymer dielectrics with hydroxyl groups how to influence OTFTs operation in ambient air.
In this thesis, we infer hydroxyl groups of PVP dielectric absorb much water vapor, and enhance extra holes in channel due to electron traps formation with dielectric surface and water vapor. Further, in order to make sure moisture could significantly affect OTFTs operation due to hydroxyl group charging. We fabricate a novel porous OTFT for water vapor could rapidly pass in and out PVP surface.
It is reported that breath NH3 concentration is higher in cirrhotic patients (0.745 ppm) than that in normal person (0.278ppm). OTFTs as NH3 gas sensors were reported to have sensitivity in the low-ppm levels. In this thesis, electron interactions rapidly with ammonia gas molecules and dielectric surface induce extra carrier in the channel by the porous structure. A novel ammonia gas sensor is demonstrated to realize a
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